The number of consumer electronic video devices which utilize some form of video display has increased in recent years. For example, it is common to use a television or computer monitor to display a signal from a video cassette recorder (VCR) or a digital video disc (DVD) player. Further, it is also common to modulate a baseband television signal generated by the VCR to form a single channel signal (e.g. a channel 4 signal) and to send the single channel signal from a consumer electronic video device to multiple display devices throughout a home or office.
Typically, a specific channel on a television receiver, and a corresponding frequency, is utilized to view a video signal from a consumer electronic video device. Conventionally, in the United States, either channel 3 or channel 4 is used to display video signals from a consumer electronic video device. Many, radio frequency distribution devices have been designed with an internal switching mechanism for switching between channels 3 and 4. Channel 3 is used in markets where channel 4 is active, and channel 4 is used in markets where channel 3 is active to prevent interference between the locally modulated signal and the active broadcast signal.
FIG. 1 provides a block circuit diagram of a conventional video distribution signal circuit (a radio frequency converter in this example) within a consumer electronic video device, such as a video cassette recorder. A radio frequency distribution signal (e.g. a baseband television signal) is applied to a terminal 100, and then is transmitted to a modulator circuit 102. The modulator circuit 102 converts the video signal to a signal that can be received by a television receiver (not shown). The circuit also includes two carrier generators 104 and 106. A switch 108 is provided to choose which carrier generator transmits its respective carrier frequency to the modulator circuit 102. For example, the carrier frequency transmitted by carrier generator 104 may correspond to channel 3, while the carrier frequency transmitted by carrier generator 106 may correspond to channel 4. When switch 108 connects modulator circuit 102 to carrier generator 106 (as shown in FIG. 1), carrier generator 106 transmits a respective carrier frequency (i.e. the carrier frequency for channel 4) to modulator circuit 102. In contrast, when switch 108 connects modulator circuit 102 to carrier generator 104, carrier generator 104 transmits a respective carrier frequency (i.e. the carrier frequency for channel 3) to modulator circuit 102.
In a typical arrangement, the carrier frequencies produced by carrier generators 104 and 106 shown in FIG. 1 overlap one another and therefore are filtered in order to meet broadcast standards. Accordingly, the circuit includes bandpass filters 112 and 114. One of the band pass filters functions to filter the signal to an appropriate frequency range for channel 3, while the second bandpass filter functions to filter the signal to an appropriate frequency range for channel 4.
For example, the carrier frequency transmitted by carrier generator 104 may correlate to channel 3, and the bandpass filter 112 may also correlate to channel 3. In such a situation, when a user switches to channel 3 (i.e. by remote control of the consumer electronic video device), switch 108 switches such that carrier generator 104 (and not carrier generator 106) transmits carrier frequency signals to modulator circuit 102, and further, switches 110 and 116 (operating in tandem, preferably with switch 108) switch such that band pass filter 112 (and not band pass filter 114) filters the signal. Conversely, the carrier frequency transmitted by carrier generator 106 may correlate to channel 4, and the bandpass filter 114 may also correlate to channel 4. In such a situation, when a user changes the VCR to use channel 4, switch 108 switches such that carrier generator 106 transmits carrier frequency signals to modulator circuit 102, and further, switches 110 and 116 switches such that band pass filter 114 filters the signal. In either of the scenarios described above, a modulated signal is produced at terminal 118 for receipt by a television receiver.
An improvement to the above-described circuit is disclosed in U.S. Pat. No. 4,213,152, due to Kakinuma, and illustrated in FIG. 2 of the present application. As described above with reference to FIG. 1, FIG. 2 illustrates a terminal 100 that receives a radio frequency distribution signal, which is then transmitted to a modulator circuit 102. The circuit also includes two carrier generators 104 and 106 which are controlled by a switch 108. As described above, the carrier frequency transmitted by carrier generator 104 may correlate to channel 3, while the carrier frequency transmitted by carrier generator 106 may correlate to channel 4.
The respective carrier frequencies overlap one another, and are filtered in order to comply with broadcast standards. In contrast to the method described with respect to FIG. 1, the simplified configuration illustrated in FIG. 2 includes a first filter 202 that is always activated, and a second filter 204 that may be activated by a switch 208. Whether or not the second filter 204 is activated depends upon which channel is being utilized (i.e. channel 3 or channel 4). Filter 202 eliminates a portion the lower sideband of the channel 3 signal while filter 204 eliminates the same portion the lower sideband of the channel 4 signal.
The systems described by reference to FIG. 1 and FIG. 2 may utilize a single cable (i.e. a coaxial cable) to transmit a signal from the video distribution device (i.e. a video cassette recorder) to a television receiver. In both examples, one of a pair of channels (i.e. either channel 3 or 4) may be used to transmit the signal to the television receiver.
Current consumer electronic video devices, such as satellite set-top boxes (IRDs) and video cassette recorders (VCRs), support either a channel 3 or a channel 4 radio frequency output to a display device (i.e. a television or a personal computer with a radio frequency input terminal). Attempts have been made to utilize a single coaxial cable to transmit radio frequency signals for two distinct channels (channel 3 and channel 4) simultaneously. For example, two satellite set-top boxes have been installed on a single network coaxial cable, with the first of the set-top boxes using a channel 3 output, and the second of the set-top boxes using a channel 4 output. However, such systems have had problems with distortion and interference between the channel 3 signal and the channel 4 signal. A primary reason for this distortion is that the consumer electronic video devices (i.e. IRDs) utilize a radio frequency output that violates the 6 megahertz bandwidth specifications of the National Television System Committee (NTSC). Because of this interference, costly external radio frequency modulators have been added to the distribution system in an attempt to move one of the IRD channels (either channel 3 or 4) to a non adjacent channel, which is usually located on the high end of the channel spectrum (e.g., in the UHF spectrum). The addition of external modulation circuitry has achieved only marginal success, and results in significant additional costs.
Therefore, it would be desirable for a radio frequency distribution network system to provide for two or more channels (each channel carrying a signal from a consumer electronic video device) along a single cable, without interference between the channels, and without the necessity for additional external modulation circuitry.